It's simple enough....when all else has melted and one just sits there untouched...it's zinc.
Scrap it off and toss it. Zinc melts at higher temps. than regular WW's do.
If you let the furnace run until the temperatures climb, eventually, the zinc will melt as well. I'm referring to the point where WW's have just melted completely and only leave the clips on the surface. At this point, any untouched WW's that are left, are surely zinc.
Take care,
Bob